Autophagy, a major lysosomal degradative pathway for organelles and long-lived or damaged proteins, Is essential for neuronal survival. We showed that neuronal autophagy is markedly defective in Alzheimer's disease (AD) and have implicated failure of lysosomal proteolysis as the key mechanism. Underscoring the pathogenic significance of autophagic-lysosomal dysregulation, we discovered that presenilin 1 (PSI) is essential for lysosomal acidification and that PS1 mutations causing AD disrupt this function. Moreover, partially restoring deficient lysosomal proteolytic function in a mouse model of amyloidosis substantially ameliorates AD-related pathology and memory deficits. Based on new evidence that APP mutations also impair autophagy, we propose In Aim 1 to define the underlying mechanism and whether or not autophagy alterations are common to APP mutations found in familial AD and App triplication in Down syndrome, and to other AD factors, e.g., cholesterol (Project 1) and cystatin C (Project 3). Contributions of upstream endocytic pathway disruption by these factors will also be explored with Projects 1, 3 and 4. In Aim 2, consequences of impaired lysosomal proteolysis on development of AD-related neuropathology and memory defects will be investigated in mouse models and dynamically in. primary neurons using video-imaging. We found that inhibiting lysosomal proteolysis induces selective retrograde transport deficits and AD-like neuritic dystrophy. The molecular basis for these effects will be defined and evidence will be sought for a similar mechanism operating in mice expressing mutant PSI or App genes. Aim 3 focuses on therapeutic interventions to prevent or reverse pathology and memory deficits In AD mouse models by enhancing lysosomal proteolytic efficiency. We identified a specific pharmacological class of agents able to modulate lysosomal pH Independently of vATPase, which will be defined in terms of mechanism and therapeutic efficacy in AD mouse models In comparison to new modulators of autophagy Induction. Our studies, the first to investigate autophagy systematically in AD, will firmly establish the pathogenic significance of autophagy dysfunction and identify innovative approaches to treat AD and other aging-related neurodegenerative disorders.